Various motion compensation techniques may be used when it is desired to calculate a frame intermediate of two frames in a video sequence. This may be necessary in slow motion simulation or in standards conversion. The positions of moving objects within these fields are calculated with reference to a plurality of the other fields in the sequence. An example of such a motion compensation system is given in our British Patent Application No. 9111348.0.
There are two commonly used motion estimation techniques used to produce motion vectors representing the differences between a pair of video images and further techniques exist.
The first of these is phase correlation and a technique for implementing this is described in our British Patent Number 2188510B and corresponding U.S. Pat. Nos. 4,873,573, 4,890,160, and 4,942,466. This produces a list of motion vectors which are applicable over the whole area of the image and over each elementary area of the image. To use the vectors it is necessary to select which of them will apply to a given elementary area of the picture. Each elementary area may be as small as a pixel or may comprise a block of many pixels.
Phase correlation has been found to work very well for linear movements in objects and is very rarely confused by detail in the scene. It has not been found, however, to be particularly effective when used on rotating objects or on scenes where small picture elements move in different directions.
The other commonly used technique for motion estimation is known as `block matching`. In this a block of video data n pixels by m pixels is compared with a corresponding block in a previous field and with each of a plurality of equally sized blocks within a predetermined range in that previous field. For a pair of interleaved video fields this range might be .+-.8 field lines vertically and .+-.16 pixels horizontally. The block which gives the best match to the original block is then used to derive a motion vector for that block.
When block matching systems are used they can easily break down in the matching stage and consequently can introduce inaccuracies when, for example, there is a camera pan.
In motion compensation systems the choice has usually been between phase correlation techniques and block matching techniques. The chosen technique has then been developed as far as possible to attempt to eliminate any failings caused by particular types of motion.